Crystal structures, phase transitions, thermodynamics, and molecular dynamics of organic–inorganic hybrid crystal [NH(CH3)3]2ZnCl4

Understanding the physical properties of organic–inorganic hybrid [NH(CH3)3]2ZnCl4 is necessary for its potential application in batteries and fuel cells due to its environmentally-friendly, and highly stable character. Here, we determine its overall properties in detail, such as its orthorhombic crystal structure, and phase transition temperatures associated with five different phases. Structural geometry was studied by the chemical shifts caused by the local field around 1H. No changes were observed for the environment around 1H for CH3, whereas the 1H chemical shifts around NH in the cation were shown due to the change in the hydrogen bond N‒H···Cl. This is related to the change in Cl around Zn in the anion. In addition, the coordination geometry of 14N and 1H around 13C exhibited increased symmetry at high temperatures. Finally, we were able to understand its molecular dynamics by the significant change with temperature observed from the spin–lattice relaxation time T1ρ values, which represent the energy transfer for the 1H and 13C atoms of the cation. The activation energies obtained from the T1ρ results were 3–4 times large at phase I (> 348 K) than at phase V and IV (< 286 K). The relaxations show that the energy barriers in phases IV and V are related to the reorientation of methyl groups around the triple symmetry axis, while the reorientation of methyl groups of the cation in phase I is related to as a whole.

In this study, the [NH(CH 3 ) 3 ] 2 ZnCl 4 single crystals were grown using the aqueous solution method, and their structures, the phase transition temperatures, and thermal properties were investigated.Nuclear magnetic resonance (NMR) chemical shifts as a function of temperature were measured to investigate the coordination geometry for the 1 H and 13 C atoms in the cations of this crystal.From these results, the N-H•••Cl hydrogen bond according to the ligands between the cations and anions was considered.And, the spin-lattice relaxation times T 1ρ representing the energy transfer around the 1 H and 13 C atoms were discussed as a function of temperature, and their activation energies E a are determined.The results of the single-crystal structure, NMR chemical shifts, and T 1ρ are predicted important information on the crystal configuration and the energy transfer mechanism for the potential applications.

Crystal growth
Single crystals of [NH(CH 3 ) 3 ] 2 ZnCl 4 were prepared from [NH(CH 3 ) 3 ]Cl (Aldrich, 98%) and ZnCl 2 (Aldrich, 98%) in a ratio of 2:1.The mixed compounds were heated to make a homogeneous solution.The mixed solution was filtered once through filter paper, and colorless single crystals grown by slow evaporation were obtained after few days in a thermostat of 300 K.

Characterization
The lattice parameters of [NH(CH 3 ) 3 ] 2 ZnCl 4 crystals were determined by single-crystal X-ray diffraction (SCXRD) at the Seoul Western Center of the Korea Basic Science Institute (KBSI).The crystal block was mounted on diffractometer (Bruker D8 Venture PHOTON III M14) equipped with a graphite-monochromated Mo-Kα (λ=0.71073Å) radiation source.Data using SMART APEX3 and SAINT was collected and integrated.The structure was solved using direct methods and refined by full matrix least-squares on F2 using SHELXTL 37 .Additionally the powder XRD (PXRD) patterns of the [NH(CH 3 ) 3 ] 2 ZnCl 4 crystals were measured using an XRD system with the same Mo-Kα used in SCXRD.
DSC measurement was performed using a DSC instrument (TA Instruments, DSC 25) with a heating speed of 10 °C/min between the 200 and 573 K temperature range under a flow of nitrogen gas.The changes of the single crystal according to temperature change were measured using an optical polarizing microscope.A hot stage was used to change the temperature (Linkam THMS 600).
Thermogravimetric analysis (TGA) was also measured with a heating speed of 10 °C/min in the temperature range between 300 and 873 K under nitrogen gas.
The magic angle spinning (MAS) NMR chemical shifts and the spin-lattice relaxation time T 1ρ of the [NH(CH 3 ) 3 ] 2 ZnCl 4 crystals were measured using a solid-state NMR spectrometer (Bruker, AVANCE III+) at the same facility, Western Seoul Center of the KBSI.The Larmor frequency for 1 H NMR experiment was 400. 13  MHz, and that for the 13 C NMR experiment was 100.61MHz.The sample was placed in cylindrical zirconia rotors and measured with a spin speed of 10 kHz for the MAS NMR measurements, in order to the reduce the spinning sideband.Chemical shifts were referenced to adamantane and tetramethylsilane (TMS) for 1 H and 13 C, respectively, as standard materials in order to accurate the chemical shift measurements.1D NMR spectrum for 1 H and 13 C was performed in the delay time of 2-20 s.The 1 H T 1ρ values were measured using π/2−τ spin-lock pulse for a duration of τ, and the π/2 pulse width was 3.65-4 μs.And, the 13 C T 1ρ values were obtained by varying the duration of a 13 C spin-locking pulse after the CP preparation period.The 13 C T 1ρ values were obtained using CP-τ acquisition.

FT-IR spectra
The FT-IR spectrum at 300 K was recorded within the 4000-500 cm −1 range.The result is shown in Fig. 1, and the peaks near 813 and 978 cm −1 are assigned to the N-C mode.And, the peak at 1253 cm −1 is related to the deformation vibration N-C-H.The bands observed 1410 and 1468 cm −1 are assigned to the CH 3 mode.The bands near 2769 and 3058 cm −1 are related to the stretching mode C-H, and the peak at 3493 cm -1 is the N-H stretching mode.This result agrees well the previously reported results of [NH(CH 3 ) 3 ] 2 CdCl 4 23 .

Crystal structure
Single crystal XRD results for the [NH(CH 3 ) 3 ] 2 ZnCl 4 crystal grown here were obtained at 300 K.The single crystal structure has an orthorhombic system with space group Pnma, lattice constants a = 10.6279(4)Å, b = 9.6297(4) Å, c = 14.9880(7)Å, and Z = 4, which are consistent with previously reported results 19 .Figure 2 shows the thermal ellipsoid and atomic number for each atom, and XRD data of [NH(CH 3 ) 3 ] 2 ZnCl 4 crystals are shown in Table 1.The infinite chains consisted of face-shared ZnCl 4 tetrahedra and four doubly bridging    www.nature.com/scientificreports/

Phase transition temperatures
The DSC thermogram for powder [NH(CH 3 ) 3 ] 2 ZnCl 4 was measured in the temperature range from 200 to 573 K with the heating rate of 10 ℃/min.Figure 3 shows the four endothermic peaks at 257 K, 286 K, 326 K, and 348 K. And, the one strong endothermic peak at 553 K was obtained.The enthalpies for the five peaks were 2.54, 7.99, 7.53, 1.86, and 17.58 kJ/mol, respectively.Starting from 200 K, these five phases were denoted as phase V below 257 K, phase IV between 257 and 286 K, phase III between 286 and 326 K, phase II between 326 and 348 K, and phase I above 348 K, shown in Fig. 3.  www.nature.com/scientificreports/ In order to know whether the five endothermic peaks shown in the DSC results shown in Fig. 3 are the phase transition temperatures or melting temperature, the changes of a single crystal were observed using an optical polarizing microscope with increasing temperature.Until the temperature rises to 530 K, the single crystal is almost unchanged, but the single crystal began to melt above 550 K.
In addition, a powder XRD experiment was measured according to the temperature change.The PXRD patterns in the range of 8°-50° (2θ) are shown at various temperature in Fig. 4. The PXRD patterns below 330 K (black) differ from that recorded above 330 K (red); this difference is related to the structural phase transition in T C2 (= 326 K).Furthermore, the XRD patterns recorded above 330 K differed from those recorded at 350 K (olive), and this difference is related to the phase transition in T C1 (= 348 K).These results are consistent with those of the DSC result.And, the theoretical XRD pattern at 300 K, which agrees well with the experimental pattern, is shown in Fig. 4.
The phase transition temperatures and melting temperature shown in the PXRD and polarizing microscope results are agree well with the endothermic peaks obtained in the DSC curve.From the DSC, PXRD, and optical polarizing microscopy results, the phase transition temperatures were determined as T C4 = 257 K, T C3 = 286 K, T C2 = 326 K, T C1 = 348 K, and the melting temperature was T m = 553 K.

Thermal property
TGA curves shown in Fig. 5 were obtained with the increasing temperature.In the TGA curve, the partial decomposition temperature T d representing a weight loss of 2 % was 495 K, and this material was thermal stable up to 495 K.The molecular weight of the [NH(CH 3 ) 3 ] 2 ZnCl 4 crystal as the temperature increased was abruptly decreased by the partial decomposition.The molecular weight losses of 11 % and 22 % calculated from the total molecular weight were by the partial decomposition of HCl and 2HCl, respectively.The initial weight loss (45 %) was occurred in the range of 500-630 K. On the other hand, an endothermic peak at 342 K appeared in the  www.nature.com/scientificreports/differential thermal analysis (DTA) curve, which is shown as a differential form of TGA, was in good agreement with the phase transition temperature T C1 shown in the DSC result.In addition, it was found that total weight loss occurred at temperatures near 800 K.

H and 13 C MAS NMR chemical shifts
The NMR chemical shifts for 1 H in the [NH(CH 3 ) 3 ] 2 ZnCl 4 crystal were recorded at phases V, IV, III, II, and I, as shown in Fig. 6.The 1 H NMR spectra for NH and CH 3 were obtained, and their sidebands for 1 H spectrum were represented as the open circles and asterisks, respectively.At 300 K, the 1 H chemical shift for NH was recorded about 7.78 ppm and the 1 H chemical shift for CH 3 was obtained about 3.21 ppm.Depending on the temperature change, there is no change near T C4 , T C2 , and T C1 , but the 1 H chemical shift for NH near T C3 shows a change.As shown inside of Fig. 6, the 1 H chemical shifts for CH 3 were hardly change as the temperature increased, whereas the 1 H chemical shifts for NH were changed, and this result means that the coordination geometry around 1 H for CH 3 does not changes according to the temperature change, whereas the coordination geometry around 1 H for NH was changes.
The 13 C NMR chemical shifts of [NH(CH 3 ) 3 ] 2 ZnCl 4 were measured in phases V, IV, III, II, and I with increasing temperature as shown in Fig. 7.In the structure of the crystal shown in Fig. 2, three 13 C atoms are bonded with 1 H and 14 N.The 13 C chemical shifts for CH 3 at 220 K of phase V showed three signals (46.98, 46.24, and 45.87 ppm), it reduced to two signals at 300 K of phase III (48.05 and 47.44 ppm), and it also reduced to one signal at 420 K of phase I (47.89 ppm).Although 14 N and 1 H are bonded around 13 C in the cation, the environments around 13 C can be different depending on the nearby Cl − .That is, the change of 13 C chemical shifts was not seen near T C4 , whereas the change of chemical shift was large in T C3 .In addition, two signals were obtained between T C3 and T C2 , and at the temperature above T C1 , only one signal was obtained.The change in the number of 13 C chemical shifts can be explained as follows.That is, in phases V and IV, there are three CH 3 with different environments, and in phases III and II, there are two CH 3 with different environments.In particular, in phase I, it was found that all the environments around 13 C in CH 3 were the same.The number of peaks in the 13 C NMR spectra decreases with increasing temperature, indicating an increase in coordination symmetry around the [NH(CH 3 ) 3 ] cations.

H and 13 C NMR spin-lattice relaxation times
To understand the spin-lattice relaxation time T 1ρ , the signal intensities of 1 H and 13 C NMR spectra were measured according to the change of the delay times.The decay curves by the change in the intensities and delay times are represented as following equation 38-40 : where P(t) is the intensity of the spectrum at time t and P(0) is the intensity of the spectrum at time t = 0.The T 1ρ values for 1 H and 13 C in [NH(CH 3 ) 3 ] 2 ZnCl 4 are obtained using Eq. ( 1), and their results are represented in Fig. 8 at phases V, IV, III, II, and I. 1 H and 13 C T 1ρ at phases V and IV increases as the temperature increases and show a maximum value near 300 K.The T 1ρ values at phase I show a tendency to rapidly decrease again.The similar tendency of the two 1 H and 13 C T 1ρ values means that 1 H and 14 N around 13 C are bonded, and 13 C and www.nature.com/scientificreports/ 14N around 1 H are bonded, so it is thought that they do the same motions.And, the fact that the 13 C T 1ρ value is longer than the 1 H T 1ρ value means that the energy transfer of 1 H bonded to the end of 13 C is easy.Near T C1 , T C2 , and T C4 , the T 1ρ values for 1 H and 13 C are more or less continuous, but T 1ρ values near T C3 are change discontinuous.To determine the magnitude of E a depending the molecular dynamics, the logarithmic scale of T 1ρ values vs. 1000/T are shown as the solid lines in Fig. 8; E a for 1 H and was found to be 10.16 ± 0.88 kJ/mol at phases V and IV.And, the E a for 1 H was 37.08 ± 2.21 kJ/mol at phase I. On the other hand, the E a for 13 C obtained from the slope of T 1ρ as the function of inverse temperature at phases V and IV was 9.80 ± 0.45 and 9.83 ± 0.42 kJ/ mol, whereas it for 13 C was 45.04 ± 1.99 kJ/mol at phase I.Although the 13 C chemical shifts are slightly different, E a at phases V and IV is almost the same within the error range.It is noteworthy that the differences in E a at phases V, IV, and I are very large.
The 1 H and 13 C T 1ρ values shows a tendency to increase rapidly at phases V and IV, whereas those shows decrease abruptly at phase I.The behaviours of the T 1ρ for Arrhenius-type molecular motions are separate into fast-and slow-motion parts.Fast motion is expressed as ω 1 τ C << 1, T 1ρ -1 ~ exp(E a /k B T), and the slow motion as ω 1 τ C >> 1, T 1ρ ~ω1 -2 exp(-E a /k B T) 38 .At the boundary of 300 K, it is divided into fast and slow motion.The 1 H and 13 C T 1ρ values at phases V and IV were in the fast-motion regime, whereas the 1 H and 13 C T 1ρ values at phase I were attributed to the slow-motion regime.

Conclusion
Investigating of the growth, phase transition temperatures, and thermodynamics of the organic-inorganic hybrid [NH(CH 3 ) 3 ] 2 ZnCl 4 crystals were considered.The orthorhombic structure of this crystal was determined by SCXRD, and the four phase transition temperatures of 257 K (= T C4 ), 286 K (= T C3 ), 326 K (= T C2 ), and 348 K (= T C1 ) were defined using DSC and PXRD results.The previously reported 19,34,35 phase transition temperatures may slightly differ depending on the crystal growth conditions.This crystal had the thermal stability of about 495 K, and weight loss resulting in the loss of the HCl and 2HCl moieties was observed with increasing temperature owing to thermal decomposition.From the chemical shifts caused by the local field around 1 H, the environments around 1 H for CH 3 does not changes according to the temperature change, whereas the environments around 1 H for NH was changes.And, the coordination geometry of 13 C becomes high symmetry as the temperature rises.As a result, the change in 1 H chemical shifts around NH in the cation is suggest be due to the change in the hydrogen bond N-H•••Cl, which is related to the change in Cl around Zn in the anion.Finally, 1 H T 1ρ and 13 C T 1ρ values, which represent the energy transfer for the 1 H and 13 C atoms of the cation are changed significantly with temperature.The activation energies considered from the NMR T 1ρ values for molecular motion were very high at high temperature of phase I than at phase V and IV.Additionally, based on the relaxation time T 1ρ measurements in the rotational system for 1 H and 13 C at different temperatures, activation barriers for the molecular reorientation of the cation in phases I, IV, and V were determined.The activation energy barriers in phases IV and V are approximately four times lower than that in phase I.The relaxations show that in phases IV and V the energy barrier was related to the reorientation of methyl groups around the triple symmetry axis, and in phase I the reorientation of methyl groups of the cation was related to the entire rotation.This work provides an understanding of the fundamental properties for applications in organic-inorganic hybrid materials.
Cl − ions linked to adjacent Zn centers.As shown in Fig. 2, this compound is connected by three hydrogen bond N-H•••Cl between the [NH(CH 3 ) 3 ] cation and the [ZnCl 4 ] anion.The bond-lengths for Zn-Cl, N-C, N-H•••Cl and bond-angles for Cl-Zn-Cl and N-H•••Cl are shown in Table2.Here, the N-H•••Cl hydrogen bond consists of an angle greater than 120°.

Figure 4 .
Figure 4. Powder X-ray diffraction patterns of [NH(CH 3 ) 3 ] 2 ZnCl 4 at phases III, II, and I.The blue colour is the theoretical powder pattern at 300 K.

( 1 )Figure 6 .
Figure 6. 1 H MAS NMR chemical shifts of NH and CH 3 in [NH(CH 3 ) 3 ] 2 ZnCl 4 at phases V, IV, III, II, and I.The open circles are sideband for NH, and the asterisks are is sideband for CH 3 (inset: the 1 H chemical shifts for NH and CH 3 near phase transition temperatures).

Figure 7 .
Figure 7. 13 C MAS NMR chemical shifts in [NH(CH 3 ) 3 ] 2 ZnCl 4 at phases V, IV, III, II, and I (inset: the 13 C chemical shifts for CH 3 as a function of temperature).

Figure 8 .
Figure 8. 1 H and 13 C NMR spin-lattice relaxation times T 1ρ in [NH(CH 3 ) 3 ] 2 ZnCl 4 at phases V, IV, III, II, and I.The slopes of lines are represented the activation energies by the T 1ρ as a function of inverse temperature.